Cubic zirconia (or CZ) is zirconium oxide (ZrO2), a mineral that is extremely rare in nature but is widely synthesized for use as a diamond simulant. The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirconium silicate (ZrSiO4).
Because of its low cost, durability, and close visual likeness to diamond, synthetic cubic zirconia has remained the most gemologically and economically important diamond simulant since 1976. Its main competition as a synthetic gemstone is the more recently cultivated material moissanite.

Contents

• 1 Technical aspects
• 2 History
• 3 Synthesis
• 4 Innovations
• 5 CZ versus diamond

Cubic zirconia is so optically close to diamond that only a trained eye can easily differentiate the two. There are a few key features of CZ which clearly distinguish it from diamond, some observable only under the microscope or loupe. For example:
• Dispersion. With a dispersive power greater than diamond (0.060 vs. 0.044) the more prismatic fire of CZ can be considered excessive and is a relatively obvious give away to even an untrained eye.
• Hardness. The inferior hardness of CZ (8.5 vs. 10 of diamond) manifests itself in the gem's lower luster, rounded facet edges and surface scratches.
• Specific gravity. CZs are heavyweights in comparison to diamonds; a CZ will weigh about 1.7 times more than a diamond of equivalent size. Obviously, this difference is only useful when examining loose stones.
• Flaws. Contemporary production of cubic zirconia is virtually flawless. Whereas most diamonds have some sort of defect, be it a feather, included crystal, or perhaps a remnant of an original crystal face (e.g. trigons).
• Refractive index. CZ has a lower refractive index than diamond.
o This allows more light to leak out of a CZ, especially when greasy or wet.
o CZ's lower refractive index causes it to have less luster than diamond.
• Cut. Under close inspection with a loupe, the facet shapes of some CZs appear different from diamonds.
o In theory, many gems (such as CZs and diamonds) look best when the star facet, crown main facets, and upper girdle facets do not quite meet. Diamond has such a high refractive index that having these facets meet at a single point does not cause much loss of fire or reflection. Diamonds normally have these facets meet at a point, because that is more symmetrical and reflects well on the cutter's precision. On the other hand, CZ has a considerably lower refractive index than diamond. CZs are often cut with 6-sided crown main facets, so that the star facets do not touch the upper girdle facets. This optimizes the brilliance and fire of the CZs.
o The optimum angle of the main crown facets is steeper for diamond than for CZ. CZs are often cut so that the crown main facets do not touch the girdle. This allows the CZs to have a shallower crown angle, while still having the same crown height as the diamonds being simulated.

Since 1892 the yellowish, monoclinic mineral baddeleyite had been the only natural form of zirconium oxide known. Being of rare occurrence it had little economic importance.
The extremely high melting point of zirconia (2750°C) posed a hurdle to controlled single-crystal growth, as no existing crucible could hold it in its molten state. However, stabilization of zirconium oxide had been realized early on, with the synthetic product stabilized zirconia introduced in 1930. Although cubic, it was in the form of a polycristalline ceramic: it was made use of as a refractory material, highly resistant to chemical and thermal (up to 2540°C) attack.
Seven years later, German mineralogists M. V. Stackelberg and K. Chudoba discovered naturally occurring cubic zirconia in the form of microscopic grains included in metamict zircon. Thought to be a byproduct of the metamictization process, the two scientists did not think the mineral important enough to formally name. The discovery was confirmed through x-ray diffaction, proving a natural counterpart to the synthetic product exists.
As with the majority of diamond imitations, the conceptual birth of single-crystal cubic zirconia began in the minds of scientists seeking a new and versatile material for use in lasers and other optical applications. Its evolution would eclipse earlier synthetics, such as synthetic strontium titanate, synthetic rutile, YAG (Yttrium Aluminium Garnet) and GGG (Gadolinium Gallium Garnet).
Some of the earliest research into controlled single-crystal growth of cubic zirconia occurred in 1960s France , much work being done by Y. Roulin and R. Collongues. The technique developed saw molten zirconia contained within itself with crystal growth from the melt: The process was named cold crucible, an allusion to the system of water cooling used. Though promising, these pursuits yielded only small crystals.
Later, Soviet scientists under V. V. Osiko at the Lebedev Physical Institute in Moscow perfected the technique, which was then named skull crucible (an allusion either to the shape of the water-cooled container or to the occasional form of crystals grown). They named the jewel Fianit, but the name was not used outside of the USSR. Their breakthrough was published in 1973, and commercial production began in 1976. By 1980 annual global production had reached 50 million carats(10,000 kg).

The Soviet-perfected skull crucible is still used today, with little variation. Water-filled copper pipes provide a cup-shaped scaffold in which the zirconia feed powder is packed, the whole contraption being wrapped with radio frequency inductioncoils running perpendicular to the copper pipes. A stabilizer is mixed with the feed powder, being typically either yttria or calcium oxide.
The RF induction coils function in a manner similar to the primary winding in a transformer. The heated zirconia acts as the "secondary winding" of a transformer which in effect is "shorted" out and thus gets incredibily hot. This heating method requires the introduction of small pieces of zirconium metal. The metal is placed near the outside of the charge and is melted by the RF coils and heats the surrounding zirconia powder from the outside inwards. The cooling water-filled pipes embracing the outer surface maintain a thin "skin" (1-2 mm) of unmelted feed, creating a self-contained apparatus. After several hours the heat is reduced in a controlled and gradual manner, resulting in the formation of flawless columnar crystals. Prolonged annealing at c. 1400°C is then carried out to remove any strain. The annealed crystals, which are typically 5 cm long by 2.5 cm wide (although they may be grown much larger), are then cut into gemstones.
The addition of certain metal oxide dopants into the feed powder results in a variety of vibrant colors. For example:
• Cerium: yellow, orange, red
• Chromium: green
• Neodymium: purple
• Erbium: pink
• Titanium: golden brown

In recent years manufacturers have sought ways of distinguishing their product by supposedly "improving" cubic zirconia. Coating finished CZs in a film of diamond-like carbon (DLC) or Amorphous Diamond is one such innovation, a process using chemical vapor deposition. This technique was developed by BetterThanDiamond.com and brought to market under the "Asha" brand name. The resulting material is purportedly harder, more lustrous and more like diamond overall: The coating is thought to quench the excess fire of CZ, while improving its refractive index, thus bringing it more in line with diamond. Additionally, because of the high percentage of diamond bonds in the amorphous diamond coating, the finished simulant will show a positive diamond signature under Raman spectroscopy.
Another technique first applied to quartz and topaz has also been adapted to cubic zirconia: Vacuum-sputtering an extremely thin layer of metal oxide (typically gold) onto the finished stones creates an iridescent effect. This material is marketed as "mystic" by many dealers. Unlike DLC, the surreal effect is not permanent, as abrasion easily removes the oxide layer.

Cubic zirconia is so optically close to diamond that only a trained eye can easily differentiate the two. There are a few key features of CZ which clearly distinguish it from diamond, some observable only under the microscope or loupe. For example:
• Dispersion. With a dispersive power greater than diamond (0.060 vs. 0.044) the more prismatic fire of CZ can be considered excessive and is a relatively obvious give away to even an untrained eye.
• Hardness. The inferior hardness of CZ (8.5 vs. 10 of diamond) manifests itself in the gem's lower luster, rounded facet edges and surface scratches.
• Specific gravity. CZs are heavyweights in comparison to diamonds; a CZ will weigh about 1.7 times more than a diamond of equivalent size. Obviously, this difference is only useful when examining loose stones.
• Flaws. Contemporary production of cubic zirconia is virtually flawless. Whereas most diamonds have some sort of defect, be it a feather, included crystal, or perhaps a remnant of an original crystal face (e.g. trigons).
• Refractive index. CZ has a lower refractive index than diamond.
o This allows more light to leak out of a CZ, especially when greasy or wet.
o CZ's lower refractive index causes it to have less luster than diamond.
• Cut. Under close inspection with a loupe, the facet shapes of some CZs appear different from diamonds.
o In theory, many gems (such as CZs and diamonds) look best when the star facet, crown main facets, and upper girdle facets do not quite meet. Diamond has such a high refractive index that having these facets meet at a single point does not cause much loss of fire or reflection. Diamonds normally have these facets meet at a point, because that is more symmetrical and reflects well on the cutter's precision. On the other hand, CZ has a considerably lower refractive index than diamond. CZs are often cut with 6-sided crown main facets, so that the star facets do not touch the upper girdle facets. This optimizes the brilliance and fire of the CZs.
o The optimum angle of the main crown facets is steeper for diamond than for CZ. CZs are often cut so that the crown main facets do not touch the girdle. This allows the CZs to have a shallower crown angle, while still having the same crown height as the diamonds being simulated.

• Color. More precisely, the lack of color: Only the rarest of diamonds are truly colorless, most having a tinge of yellow or brown to some extent. By comparison, CZ can be made entirely colorless: equivalent to a perfect "D" on diamond's color grading scale. Furthermore, the fancy colors of CZ in no way approximate the shades of fancy diamonds.
• Thermal conductivity. This is probably the most important property of diamond from a jeweller's perspective: all they need do is apply the tip of a thermal probe to a suspect diamond. CZs are thermal insulators whilst diamonds are among the most efficient thermal conductors, exceeding copper.

ZIRCONITE weighs 65% more than a real diamond. Therefore, the only way to tell
the difference between the two is by weighing
the unset stone.
Once it is set, it is impossible for even a jeweler
to tell the difference. A few years ago,
ABC’s 20/20 news team took a high
intensity cubic zirconia stone, as well as a perfect $50,000,000 mined diamond to the
New York Diamond District.
After careful examination of the two stones
an expert claimed that they were both so
perfect that they must both be counterfeit!
Item # p6732

The only difference therefore between ZIRCONITE and a diamond
lies in the price, and in the mind...